The present invention relates to scanning of color images and is more particularly directed to techniques for maintaining image quality and especially for maintaining image quality while scanning at higher speeds.
Optical image scanners convert an object to be scanned such as a printed document, photograph, transparency or other image or scene into a digital electronic signal representative of the scanned object. The electronic signal may then be subjected to further processing and analysis and sent to an output device such as a printer or display monitor.
The image is typically captured by a sensor responsive to light from the target. For gray scale images (that is, so-called xe2x80x9cblack and whitexe2x80x9d images) a sensor is used that is responsive only to the luminance of the light from the scanned object and that does not distinguish colors. For color images a sensor is used that is separately responsive to the red, blue and green primary color components of the light from the target.
The sensors partition the image into arrays of pixels and associate a luminance value or red, blue and green values with each pixel. Commercially available gray scale sensors are generally capable of performing at higher speed and higher resolution than commercially available color sensors. Gray scan sensors typically operate in the range of 10 to 100 megapixels per second (MPix/s) while color sensors operate at 1 to 20 MPix/s. In comparing color and gray-scale rates, one color pixel is considered to have three color data channels associated with it for the three primary colors. Color scanners perform more slowly in part because of the increased amount of data they must scan and process.
In an attempt to improve the scanning speed of color images and improve image quality, some scanners include both luminance sensors and color sensors and scan the luminance data and the color data at two different resolutions. See for example U.S. Pat. Nos. 5,045,932 and 5,619,590. A high-resolution luminance sensor is used to capture the image detail, and a lower-resolution color sensor captures the color information. The data from the two sensors are then combined according to an appropriate scheme to provide an output signal. While such schemes generally show an improvement in scanning speed, and are sometimes able to maintain good image quality, they nevertheless represent a compromise in the quality of the original image.
The present invention provides a color scanning technique that maintains a high image quality while permitting improved scanning speed and improved perceived resolution. It is an object of the invention to achieve high-resolution color scans and particularly to improve upon the resolution that has generally been realized from commonly available color charge-coupled device (CCD) sensors. It is another object of the invention to achieve higher effective scanning speeds than has generally been realized with the commonly available color CCD sensors. It is yet another object of the invention to provide improved color coordinates, that is to say, values associated with each pixel, encoding the color information for each pixel. An important aspect of the invention is that the luminance data channel determines the overall luminance of the combined output signal, while the color data channels supply only the color information.
Briefly, a target object is scanned in accordance with the invention by generating a color pixel array representative of the target object, or at least of the portion of the object of interest, and also generating a luminance pixel array representative of the same portion. The color pixel array and luminance pixel array will generally be different, but they are related in that one or more luminance pixels of the luminance pixel array cover each pixel of the color pixel array. A luminance value is sensed for each luminance pixel, and three primary color values are sensed for each color pixel. A measured luminance value is then associated with each respective color pixel wherein the measured luminance value is a function of the sensed luminance values for the one or more luminance pixels covering the respective color pixel. In addition, a derived luminance value is also calculated for each respective color pixel wherein the derived luminance value is a function of the three sensed primary color values for the respective color pixel. A luminance correction factor is then determined for each respective color pixel as a function of the color pixel""s derived and measured luminance values. The luminance correction factor for each respective color pixel is applied to the sensed primary color values of the color pixel, or to a linear combination thereof, to determine luminance-corrected color values. In this way the overall pixel luminance implied by the aggregate measured color values will show a consistency with the directly measured luminances that has not heretofore generally been realized or appreciated in the scanning field. The luminance-corrected color values together with the measured luminance values may then be subjected to further appropriate transformation, if desired, to determine color coordinates for each luminance pixel and/or for each color pixel. The color coordinates so determined provide an improved representation of the target object image exhibiting improved image quality, which may then be used to advantage in further customary image processing functions such as image compression, filtering and analysis, storage, transmission, printing or display.
In practice, determining and applying the luminance correction pixel by pixel can place a large computational burden on the system, which could appreciably slow down the processing. This comes about because the determination and application of the luminance correction factor will generally require pixel-by-pixel multiplication and division operations, which are demanding on system resources. The computational burden is greatly reduced, however, and the system processing rate is maintained, by converting the computationally laborious multiplication and division operations to much simpler addition and subtraction operations performed on the logarithms of the appropriate quantities.
The invention may advantageously be practiced with luminance and color sensors of high scanning rates so that the overall system will exhibit a high scanning rate. In so doing, the luminance correction of the present invention will provide a superior image quality to that achievable at the same scanning rate without the benefit of the invention. Moreover, a high-resolution image with improved image quality may be achieved with a high-resolution luminance sensor generally of higher resolution than the color sensor. Even with luminance and color sensors of same resolution, i.e., luminance pixels and color pixels of the same size, an improvement in image quality still results because the color values are corrected to be consistent with the true overall luminance value.
Other aspects, advantages, and novel features of the invention are described below or will be readily apparent to those skilled in the art from the following specifications and drawings of illustrative embodiments.